U.S. Pat. No. 4,641,244 to Wilson et al. discloses a method and apparatus for registering color separation film which has many advantages over the manual registration process that had been prevalent in the past. However, the invention disclosed therein has practical limitations which make it useful principally in situations where the pictures have similar digital tones. In recognition of these limitations, an edge based registration system has been developed and disclosed in pending application Ser. No. 099,800, filed Sept. 22, 1987 by Medioni et al. and entitled "Method and Apparatus for Registering Color Separation Film". The edge based system has been found to be advantageous in a number of respects and functions in a satisfactory manner for the most part. However, it has also been found to be subject to problems.
Perhaps most notably, the accuracy of the edge based system is unacceptable in some situations, as when sharp edges are present on the pictures. Typically, the sharpest edges are found in reverse type which is used, for example, in the lettering in four color magazine advertisements. The registration accuracy in this situation is typically about 0.75-1.0 mil, which is to be compared with the accuracy of less than 0.5 mil that can be achieved manually by stripping professionals. Sharp edge registration in general and reverse type registration in particular are of considerable importance in the printing industry because sharp edged detail in a printed publication is rather prominent if there are noticeable registration errors. Consequently, even though registration errors up to 1 mil in ordinary (non-sharp) edge detail are difficult to detect visually and thus acceptable, registration errors greater than 0.5 mil are objectionable in sharp edged detail. Thus, the registration errors in sharp edged detail that result from use of the invention disclosed in pending application Ser. No. 099,800 are unacceptable.
Because the registration schemes used in prior methods are based on only two registration points, the entire film images may not be registered in some circumstances even if both registration points are exactly registered. For example, if the film image is stretched (global image size difference) or if multiple images are placed on a composite film and each such image is placed out of registration by a random error introduced by human error in manually composing the final film (known as "internal misregister") the different color separations may be out of register at some points even though two registration points are in registration. Films that are subject to either image stretching or internal misregister or a combination of the two are referred to as "problem films", and such problem films cannot be accurately registered by two point registration techniques.
A third problem with the edge based technique disclosed in Ser. No. 099,800 is that halftOne images with blurred or fuzzy edge detail are rejected more often than necessary by the quality control check that is used. In order to be practical, the registration technique should be more forgiving as to registration of fuzzy halftone pictures.